Hypertransparent Nanostructured Superhydrophobic and Surface Modification Coatings

ABSTRACT

Hydrophobic and self-cleaning surfaces have wide applications, including glasses, camera covers, windows, solar panels and high-end finished surfaces. Many existing hydrophobic coatings either have low transmittance, making them unsuitable for high light transmission applications, or are insufficiently hydrophobic. The present invention concerns high-quality hypertransparent superhydrophobic coatings, for example SiO 2 -based, with double-roughness microstructure that were deposited on to, for example, glass substrates using, for example, the combustion chemical vapor deposition (CCVD) technique. Embodiments of the invention include coatings with a contact angle of higher than 165°, a rolling angle of &lt;5°, a haze of &lt;0.5%, and an increased transmittance by 2% higher and a reflectance of 2% lower than bare glass. The double roughness can improve wear resistance. Additionally, other surface chemistries can be applied to yield hydrophilic, oliophobic, or oliophobic surfaces.

CROSS REFERENCE TO RELATED CASES

This application claims the benefit of priority of U.S. Provisional Application Ser. No. 61/151,358, filed on Feb. 10, 2009. The entirety of that provisional application is hereby incorporated.

STATEMENT OF FEDERAL SUPPORT

This work was supported by the Department of Energy (DOE), through Grant No. DE-FG02-04ER84007. The United States Government has certain rights in the invention.

BACKGROUND TO THE INVENTION

Studies of superhydrophobic self-cleaning surfaces have attracted increasing interest in recent years as a result of numerous new prospects for both fundamental research and practical applications. The applications of self-cleaning surfaces include architectural glass for homes and commercial buildings, automotive glass, shower doors, solar panel glass covers, nanochips, and microfluidic systems (Nakajima et al., Chem. Monthly 132, 31, 2001; Patankar, Langmuir 20, 8209, 2004; Quere et al., Nanotechnology 14, 1109, 2003).

With wide usage of self-cleaning surfaces, over $100 million a year in energy savings could result, by reducing or removing the need for washing, scrubbing, and chemical polishing of windows, ceramics, and other surfaces. In addition to a reduction in cleaning requirements, these superhydrophobic surfaces have additional benefits, such as improved safety when driving in rain and snow, and improved efficiency of solar cells.

The development of superhydrophobic self-cleaning surfaces was first inspired by the observation of natural cleanness of lotus leaves (Barthlott & Neinhuis, Planta 202, 1, 1997) and other plant leaves (Feng et al., Adv. Mater. 14, 1857, 2002). A superhydrophobic self-cleaning effect was observed in nature in lotus leaves. The lotus is revered as a symbol of purity for its ability to maintain clean leaves even in contaminated waters.

In studies of lotus leaves by scanning electron microcopy (SEM), it was revealed that the key features of the lotus leaf are a microscopically rough surface, consisting of an array of randomly distributed micropapillae with diameters ranging from about 5 to about 10 μm (Feng et al., Adv. Mater. 14, 1857, 2002; He et al., Colloids and Surfaces A: Physicochem. Eng. Aspects 248, 101, 2004). These micropapillae are covered with waxy hierarchical structures in the form of branch-like nanostructures with an average diameter of about 125 nm.

Motivated by the nature and the superhydrophobic self-cleaning performance of the lotus leaf, many techniques have been being developed to create nanostructures mimicking the lotus effect from many materials, both in organic and inorganic (Wu et al., Chem. Vap. Deposition 8, 47, 2002; Shang et al., Thin Solid Film 472, 37, 2005). However, existing hydrophobic coatings either have low transmittance, which is not suitable for windows and solar panels, or are not as hydrophobic as lotus leaves.

In the present invention, an open atmosphere combustion chemical vapor deposition (CCVD) technique was employed to deposit SiO₂-based superhydrophobic coatings onto glass substrates that provide increased light transmission and are thus hypertransparent. The coatings' morphological, hydrophobic, and other physical properties are described. The structure of the film is most important and the composition can be varied. High quality results have been achieved with glass modifiers with silica. Alternate materials to silica can also be used as long as the right nanostructure is achieved and the surface will interact with the fluids as desired or an interface modifier is used so that the correct fluid interaction is achieved. Hydrophilic and oil-interactive surfaces can also be made on the nanostructure of the present invention, by modifying the surface wetting properties.

The nanocoatings of the present invention can be applied to any surface in which optical properties are important, in addition to modifying the surface properties. Examples products that might use such nanostructured surfaces include, but are not limited to, vision wear, lenses or lens covers, windows, photovoltaic devices, and many other glass and clear plastics applications, painted surfaces, signs, transportation vehicle surfaces, architectural finishes, laboratory and food service areas, consumer products, manufacturing facilities, and both salt and freshwater interfaces, such as boat surfaces. Essentially any product or surface on which it is desired to have a good appearance and especially those where reduced maintenance or cleaning are desired may be appropriate. For most applications, water is the primary liquid interaction medium, but in others it can be oil. In such oil cases, oliophobic or oliophilic surfaces can help maintain the cleanliness or desired functionality. The interacting oil can be synthetic, fossil, or natural in source.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1. Schematic representation of a CCVD system.

FIG. 2. SEM image of an example CCVD SiO₂-based superhydrophobic coating on a glass substrate.

FIG. 3. Contact angle of the SiO₂-based coatings as a function of surface roughness.

FIG. 4. Optical images of water droplets sitting on (a) bare glass substrate and (b) an example fluorinated silane-coated CCVD SiO₂ coating on a glass substrate.

FIG. 5. Haze of example SiO₂-based coatings as a function of surface roughness.

FIG. 6. Transmittance and reflectance of an example SiO₂-based superhydrophobic coating on glass.

FIG. 7. (a) CA and (b) CA hysteresis of example CCVD SiO₂ coatings deposited at different processing conditions as a function of processing motion speed. Each symbol indicates a change in processing conditions.

FIG. 8. Contact angle as a function of abrasion pass of A and B on processed example CCVD coatings on glass.

FIG. 9. Examples of some of the materials that can be coated. The upper image is uncoated and the lower image is coated.

FIG. 10. In a preferred embodiment, an undulating surface that is dense and hard has the nanostructure (not shown in this image) located primarily below the high points of the dense material. This figure illustrates an example of such an undulating substructure, grown using CCVD. The undulating surface can help impart abrasion resistance.

EXAMPLES

Superhydrophobic Coatings by the CCVD Process

Superhydrophobic and other surface modifying coatings have been grown by many techniques, such as CVD (Wu et al., Chem. Vap. Deposition 8, 47, 2002) and sol-gel (Shang et al., Thin Solid Film 472, 37, 2005). These techniques can require costly starting materials, be time consuming, and/or have low throughput.

The inexpensive CCVD technology offers an attractive way to grow nanostructured superhydrophobic coatings on glass and plastic substrates with good yield and high throughput potential. While CCVD is the preferred method, other techniques, such as various vapor deposition methods, may be used to achieve the structures of the present invention. SiO₂ was chosen as an example primary coating material because of its low cost, ease of preparation, and the refractive index match between the coating and many candidate substrates, which reduces reflectivity of the coated specimens.

In the CCVD process (Hunt et al., App. Phys. Lett. 63, 266, 1993), as shown in FIG. 1, precursors, which are metal-bearing chemicals used to coat an object, are dissolved in a solvent, which typically also acts as the combustible fuel. This solution is atomized to form submicron droplets, using, for example, the proprietary Nanomiser device. These droplets are then entrained in an oxygen-containing stream to the flame where they are combusted. A substrate (the material to be coated) is coated by simply drawing it over the flame plasma. The heat from the flame provides the energy required to evaporate the ultrafine droplets and for the precursors to react and to vapor deposit on the substrates. The CCVD technique can use a wide range of inexpensive, soluble precursors that do not need to have a high vapor pressure. Key advantages of the CCVD technique include open-atmosphere processing, high quality at low cost, wide choice of substrates, and continuous production capability.

As examples, the coatings illustrated here were fabricated by the CCVD technique. The surface should be cleaned prior to coating; there are many known methods for achieving this. In these examples, prior to deposition, glass substrates were ultrasonically cleaned in organic solvents, such as isopropanol, rinsed in deionized water, and blown dry using nitrogen. The substrate was then mounted on a metal chunk or the top of a back heater. The CCVD process was used to achieve a nanotexture surface with a decreasing volume fraction of solid material away from the substrate surface. Important process parameters include deposition temperature, solution concentration, motion speed, and coating thickness. The surface temperature, motion speed, and concentration need to be high enough to not grow a dense structure but not so high as to make a powdery, poor adhesion layer. The best conditions depend on the coating composition and substrate properties. Polymers require lower temperatures than most inorganics. Surface temperatures in the range of about 50 to about 500° C. are generally best for silica-based coatings, with solute concentrations of about 30 to about 200 mM. The higher the temperature, the faster the motion needs to be or the coating structures can grow in size, so there is a wide range of motion speeds from near stationary to 200 m/min or higher. If a low interaction with light is desired, then the nanostructured coating thickness should preferably be less than about 300 nm, more preferably less than about 200 nm, and most preferably less than about 100 nm.

When depositing a silica-based layer, the coating is normally very hydrophilic. To increase water contact angle (CA), the coated specimens were surface-treated with a chemical to change the silica from hydrophilic to hydrophobic. There are many known ways for achieving this. In the examples, samples surfaces were made hydrophobic using fluorinated silane, by immersing the specimens in a hexane solution of the fluorinated silane (with a volume ratio of silane to hexane of about 1:50) for about 10 min. The specimens were then rinsed by hexane and deionized water and blown dry by nitrogen gas. Alternate surface chemistries can be used to for hydrophilic, oliophobic, or oliophilic surface; such surface modifying chemistries are known to those of skill in the art.

Analytical Techniques

Equilibrium, receding, and advancing CAs were measured by a CA measuring system (G10, Kruss USA). Equilibrium CAs were measured using deionized water droplets of approximately 1-2 mm in diameter. Unless indicated specifically, all the CAs in the following sessions are equilibrium water CAs. To measure advancing CA, a water droplet of about 1-2 mm in diameter was first placed on the film surface. The cursor line was placed in front of the water droplet. The droplet was then enlarged by pushing more water through the needle. While the droplet volume increases, the contact line between water and the solid surface moved forward. The advancing CA was measured once the droplet front reached the cursor line. To measure receding CA, a large water droplet of about 5 mm in diameter was placed on the solid surface first. The cursor line was placed between the syringe needle and the droplet front. The droplet volume was decreased by sucking water back into the syringe. When the contact line began moving and the droplet front reached the cursor line, the receding CA was recorded. During the measurements of advancing and receding CAs, the syringe needle was always in the water droplet. Three data points were tested on most samples.

The coating's morphology was observed by SEM (Hitachi s-800). Transmittance and reflectance in the visible range were measured by a spectrometer (Perkin-Elmer Lambda 900 UV/VIS/NIR spectrometer). Surface roughness (as root mean square, RMS) was evaluated by an optical profilometer (Burleigh Instruments, Inc.). This optical profilometer measurement is consistent, but can understate the roughness of nanostructured layers due to their low interaction with light. Haze in the visible range was characterized by a haze meter (BYK Gardner Haze Meter).

To achieve low haze and high transmittance, the feature size of the coating should be much smaller than the visible wavelength, to reduce large light scattering. FIG. 2 shows an SEM image of an example SiO₂ coating on glass. The coating has a rough surface and double surface roughness, in which coarse features are composed of nanostructures of about 30 to about 200 nm, with even smaller end nanostructures below the resolution of the SEM used. These structures are not fully dense, and are believed to go smaller than 10 nm and even 5 nm in dimensionality. The finer the nanostructures, generally, the more susceptible they are to wear. To impart better wear resistance, larger dense structures can be grown, with the high-surface-area material remaining in the low areas, protected by larger structures. The multiple roughness morphology has similarity to the topology of the lotus leaf. The nanometer sized hierarchical structure is important in simultaneously achieving low haze, increased transparency, and superhydrophobicity. The large dense structures can have more haze and reflectance before the nanostructure is grown on it.

Nanostructured SiO₂-based superhydrophobic coatings have been deposited, for example, on to ceramics, metals, glass, and plastics. Dopants, including B, P, and A1 and combinations of any two of the elements or all three elements, have been used to improve the physical properties of the superhydrophobic coatings, especially their durability. Other processes, such as in situ annealing by a solvent flame or post-deposition heat treatment in an oven, have also been employed to improve the coatings' physical properties, especially their durability.

Additionally, nanostructured ZnO-based nanostructured coatings, with or without an Al dopant, have also been deposited on to different substrates, such as, for example, porous ceramics and glass. On top of the ZnO coating, a thin layer of SiO₂, either continuous or isolated islands, is deposited to modify the surface chemistry of the ZnO-based coating for surface treatment by fluorinated silane(s) to form superhydrophobic coatings. Without the thin SiO₂ layer, the ZnO based nanostructures are still hydrophobic after silane treatment. However, the hydrophobicity is not as high as the ZnO-based coatings with a thin SiO₂ layer. Other similar processes, such as in situ annealing or post-deposition annealing, can be used to improve the coatings' durability. Different surface treatment chemicals can have different interactions, based on the chemical composition of the nanostructured layer.

The relationship between CA and surface roughness of example SiO₂ coatings is presented in FIG. 3. It is clear that CA increases rapidly with the increase of surface roughness up to about 1.5 nm. When surface roughness reached about 1.5 nm or higher, CA remains almost constant. A CA of over 165° was achieved with surface roughness ranging from about 1 to about 20 nm. The surface roughness was measured using optical profilometry; this cannot resolve all of the smallest structures, but does provide a measurement of general change in surface topology. For reference, the bare glass substrate coated with the same fluorinated silane has a CA of approximately 110°, with a surface roughness of about 0.70 nm. For comparison, photographs of water droplets sitting on bare and silane/SiO₂-CCVD coated glass are shown in FIG. 4. Water droplets spread out on bare glass with a CA of less than 20°, showing the hydrophilic nature of the glass (FIG. 4( a)). After coating with an example nanostructured SiO₂ film, the surface is even more hydrophilic until the surface is treated with the fluorinated silane, water droplets bead up to form spheres, showing superhydrophobicity (FIG. 4( b)).

Haze of the superhydrophobic coatings is another important property for many applications. For comparison. The haze of the bare glass is about 0.2%. As shown in FIG. 5, when measured surface roughness is less than about 2 nm, haze increases slightly with increasing roughness. Beyond an optical profilometry-measured roughness of about 3 nm, haze increases almost exponentially with increasing roughness. For samples with surface roughness less than 2.0 nm, a haze of less than 0.5% was obtained with a contact angle of over 165° and a CA hysteresis of less than 5°.

FIG. 6 shows typical transmittance and reflectance spectra of one side CCVD SiO₂-coated and bare glass substrates in the visible range. The transmittance and reflectance of bare glass are in the range of 91.8 to 92.6% and 7.4 to 8.5%, respectively while those of the example CCVD SiO₂-coated samples are in the range of 93.9 to 94.5% and 5.6 to 6.2%, respectively. The SiO₂-coated glass is hypertransparent because it has increased transmission, by about 2%. Reflectance is about 2% lower than the bare glass substrate, suggesting that the CCVD SiO₂ coatings reduce reflection in the visible range and are anti-reflective, which will be of benefit in many applications, such as solar cells, lighting, and low lighting vision and imaging.

Process parameters, such as deposition temperature, deposition time/lap, and flame motion speed were investigated with regard to their effects on hydrophobicity. FIG. 7 shows the CA and CA hysteresis of example SiO₂ coatings deposited at a certain solution concentration and different processing conditions as a function of motion speed. As shown in FIG. 7( a), all other samples maintain almost the same CAs with the increase in motion speed. CA hysteresis, which is directly related to rolling angle, is another important factor in evaluating superhydrophobic self-cleaning surfaces. The smaller the CA hysteresis is, the easier the water droplets roll off the surface; that is, the higher the self-cleaning performance. FIG. 7( b) shows the effects of motion speed on CA hysteresis as a function of deposition variables including one or two laps. All samples but one show nominal change in angle hysteresis with the increase of motion speed.

Abrasion resistance is an important factor for many practical applications of self-cleaning surfaces. Abrasion tests were conducted by moving the samples across a defined distance on a polishing cloth surface. Force was determined by the weight of the samples themselves. CA was measured after each two passes across the abrasion surface. In total, twenty passes were completed for each sample. FIG. 8 shows abrasion test results of two differently processed example CCVD SiO₂ coatings. The initial CA of the A-processed coating was 170°. It decreased rapidly in the first ten passes of abrasion. After ten passes, the contact angle remained almost constant at 150°. It was noticed that after the abrasion test, the rolling angle increased significantly. To increase the strength of the superhydrophobic surfaces, a sample was differently processed (B-process), then coated with silane. As shown in FIG. 8, its initial CA was 168°, about 2° lower than that of the A-processed one. In the first four passes, the CA decreased rapidly, to 159°. After four passes, the CA decreased minimally. After 20 passes, the final CA was 157°, which is 8° higher than that of the A-deposited sample, suggesting that B-processing increased the coating's strength. The B-processing resulted in a stronger base structure, more resistant to abrasion.

To further enhance abrasion resistance, even larger microstructures can be grown or formed. FIG. 10 is an example of such a substructure that was grown using CCVD. These 100 nm-to-multiple-micron structures can also be made by other deposition methods and roughening techniques, including chemical etching. In a preferred embodiment, an undulating surface that is dense and hard has a finer nanostructure coating located primarily below the high points of the dense material. The undulating surface of FIG. 10 is composed of a fracture tough material, YSZ, that could further help wear resistance, but other hard materials, including silica, can be used. Preferably, at least 90%, and more preferably at least 99%, of the surface area is nanostructure, versus the larger wear-resistant material. Most of the nanostructure though should be at or below the peaks of the dense material peaks; preferably more than 90% and more preferably 99% is lower than the tops of the dense wear-resistant material peaks. One method for achieving this nanostructure in the valleys is to first form the dense undulating surface, coat this with the nanostructure, and then abrade the surface to remove that part of the nanotexture that is above the dense material. After removing the unprotected nanostructure, then any desired surface modifying chemistry can be applied.

There are also numerous surfaces that have a finish that are desired to be viewed or have functionality in its optical properties. Painted, polished or mirrored surfaces are just a few examples. The present invention can be used to impart hydrophobic properties on these without altering or actually improving the viewing or performance of the surface. Without surface treatment silica is generally hydrophilic. This can also be desired in anti-fogging and other such applications. Instead of modifying with a hydrophobic surface chemistry one can use oliophobic or other desired surface functionality.

In summary, these examples demonstrate embodiments of the present invention; hypertransparent superhydrophobic surfaces with high performance were prepared on glass substrates by the CCVD technique. A contact angle of 170°, a rolling angle of <5°, a haze of <0.5%, and a transmittance 2% higher and a reflectance of 2% lower than bare glass were achieved. Alternatively, hydrophilic surfaces can be created with contact angles of less than 10° and can get to immeasurably low angles. Oliophobic surfaces above 140° can be achieved, as can oliophilic surfaces of less than 10°. To our knowledge, there is no previous report of others obtaining a substrate with similar or improved optical properties, compared with uncoated substrate, along with such strong hydrophobic properties. 

1. A product with a nanostructured inorganic-based hypertransparent interfacial modifying coating on at least one surface, with a light transmission or reflection similar to or higher than that of the uncoated surface.
 2. The product of claim 1 where said coating is a hydrophobic coating, with a contact angle greater than 130°.
 3. The product of claim 1 where said coating is a nanostructured inorganic-based hypertransparent hydrophobic coating, with a contact angle greater than 145°.
 4. The product of claim 1 where said coating is a nanostructured inorganic-based hypertransparent hydrophobic coating on glass, with a contact angle greater than 160°.
 5. The product of claim 1 where said coating is a nanostructured inorganic-based hypertransparent hydrophobic coating on glass, with a contact angle greater than 170°.
 6. A method using a vapor deposition technique for making the coating of claims 1, 2, 3, 4, or
 5. 7. The coating of claim 1 wherein said nanostructured inorganic-based hypertransparent interfacial modifying coating is silica-based.
 8. The products of claim 1 wherein light is required to pass through said coating.
 9. The products of claim 1 wherein the light transmission through said coating is increased at least 1% compared to the substrate prior to coating.
 10. The products of claim 1 wherein the light transmission through said coating is increased at least 2% compared to the substrate prior to coating.
 11. A method for coating a substrate with a nanostructured SiO₂-containing layer such that the substrate has similar or higher light transmission than the uncoated substrate and haze of less than 5%.
 12. A method of claim 11 where combustion chemical vapor deposition (CCVD) is used to coat the substrate.
 13. A coating prepared by the method of claim 12 such that the substrate has >1% higher light transmission than the uncoated substrate.
 14. A coating prepared by the method of claim 12 such that the substrate has >3% higher light transmission than the uncoated substrate.
 15. The coating of claim 1 which is wear-resistant surface with at least 90% of the surface area being nanostructured and the surface having an undulating surface of denser, more abrasion-resistant material.
 16. The wear-resistant surface of claim 15 with at least 99% of the surface area being nanostructured and the surface having an undulating surface of denser, more abrasion-resistant material.
 17. The coating of claim 13 which is wear-resistant surface with at least 90% of the surface area being nanostructured and the surface having an undulating surface of denser, more abrasion-resistant material.
 18. The wear-resistant surface of claim 17 with at least 99% of the surface area being nanostructured and the surface having an undulating surface of denser, more abrasion-resistant material.
 19. The surface of claim 1 where the surface is oliophobic with at least 130 degree contact angle to many oils.
 20. The surface of claim 1 where the surface is hydrophilic with at less than 10 degree contact angle.
 21. The surface of claim 1 where the substrate is not transparent.
 22. The surface of claim 19 where the surface is also hydrophobic to at least 130 degrees contact angle. 